282 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



of growth, thus creatinoj a greater diversity of size in the apparently mature 

 shell than exists in any other species. From the nucleus until the accomj)lish- 

 ment of five full whorls, each whorl on the base is curved a little lower than 

 that which precedes it; and up to this time, consequently, the umbilicus is 

 deep and gradually expanding, exhibiting, when carefully examined, all the 

 volutions. Up to this period, also, the spire is almost always prominent. 

 After five whorls are completed, the succeeding ones usually fellow in the same 

 horizontal plane, and give a discoidal character to the shell. It is manifest, 

 therefore, that specimens in each of these stages must present considerable dif- 

 ferences ; and, accordingly, the small, delicate shell, having a slightly convex 

 spire of five whorls, a deep umbilicus, and a transverse diameter of only one 

 eighth of an inch, forms a beautiful variety, and has been thought to be a dis- 

 tinct species. 



The form known as volvoxis is found on the Atlantic coast of Florida and 

 Georgia. It is thus described by Pfeilfer. The synonymy is also given in 

 full. I believe it to be a variety of septemvolva : — 



Shell umbilicated, orbicularly convex, thin, reddish horn-colored, pellucid, 

 with regular rib-like striaB ; spire very short, convex ; whorls 7, convex, regu- 

 larly increasing, the last larger above than the rest, angular, below the angle 

 inflated, striated, and shining ; umbilicus large, regular, in which the whorls 

 regularly decrease, excepting the last, which is very broad ; aperture rather 

 large, kidney-shaped ; peristome thickened within, reflected, its terminations 

 joined by a short, triangular, tooth-like callus. Greater diameter 9, lesser 

 8 mill. ; height, 4 mill. 



Jlelix volvoxis, Parreyss in Pfeiffer, Symb., III. 80 ; ^lon. Hel. Viv., I. 409 ; 

 in Chemnitz, ed. 2, I. 379(1846), PI. LXVI. Figs. 4-6 (1849). — Reeve, 

 Con. Icon., No. 1237 (1854). —W. G. Binney, Terr. Moll. U. S., IV. 92, PL 

 LXXVIII. Fig. 17. —Bland, Ann. N. Y. Lye, VII. 135. 



Polygyra volvoxis, Tryon, Am, Journ. Conch., III. 159, PL XI. Fig. 25 (1867). 



Jaw long, narrow, slightly arched ; ends attenuated, bluntly rounded ; ante- 

 rior surface with 7 stout, distant ribs, crenulating the cutting edge. 



There are 28—1—28 teeth, with 9 laterals on the lingual membrane of the 

 large form (PL VI. Fig. L). The small form with 5 whorls differs only in 

 having somewhat fewer teeth. The form known as volvoxis does not differ 

 excepting in having fewer marginals ; Jacksonville, Florida, specimens have 

 20—1—20 teeth. 



The Museum of Comparative Zoology at Cambridge has a reversed speci- 

 men of P. septemvolva. 



Plate XV. Fig. H represents the genital system of the large form of this 

 species. It is characterized by its extreme length, as would be expected from 

 the form of the shell. The vagina is extremely long and narrow. The genital 

 bladder is elongated oval, on a short, slender duct. The penis sac is very 

 long, attenuated to a point above, where the retractor muscle is inserted. 



